(a) Field of the invention
The present invention relates to a field of view direction changing optical system to be used with an optical apparatus such as an endoscope, a periscope and the like.
(b) Description of the prior art
An optical apparatus having a long and slender image transmitting optical system such as of an endoscope, a periscope and the like cannot be made larger than a certain degree because the outer diameter of the apparatus is generally limited, specially the outer diameter of the endoscope is 2 cm at most even with a large one and is 2.about.3 mm with a slender one to limit the diameter to be extremely thin. And, in such an optical apparatus, a prism is generally used for the field of view direction change necessary to effect a slant view, a side view and the like, but the field of view direction change under the circumstances in which the outer diameter is limited as mentioned above is accompanied with considerable difficulties. In the case of the side view, for example, it is comparatively easy because it can be realized only by arranging a right angle prism in front of an objective lens, but in the case of the slant view it was absolutely impossible to be assembled in an endoscope and the like because a prism 1 positioned in front of an objective lens 2 is necessarily very large as shown in FIG. 1A or FIG. 1B. By the way, 3 is an image guide.
Therefore, in order to settle this problem, the endoscope, for example, disclosed in Japanese Patent Application Laid-Open No. 50-91333 is so formed that the prism 1 is divided as shown in FIG. 2 into a first prism 1a and a second prism 1b of which an incident face S.sub.1 is used as a total reflection face and the light entered from the incident face S.sub.1 into the second prism 1b is again reflected on the incident face S.sub.1 after having been reflected on the reflecting face S.sub.2 so as to advance to an image transmitting optical system 3. However, such a field of view direction changing optical system has a defect such that the image deteriorates due to the occurrence of astigmatism because the luminous flux necessarily has to pass obliquely through the air space between the first prism 1a and second prism 1b while it has an advantage such that the prism portion can be made small. Further, as the exit face S.sub.4 is inclined with respect to the incident face S.sub.3 of the first prism 1a, the assemblage is very troublesome because a high accuracy is requested in an adjustment to eliminate an aberration of the relative position between the first prism 1a and the second prism 1b or the position, angle and the like to fix the both prisms 1a and 1b in the main body of an endoscope. That is to say, as well known, it is because inclinations and eccentricities of respective elements in the optical system generate the deterioration of astigmatism.